Gas diffusion electrode and method for its production

ABSTRACT

A gas diffusion electrode for a PEM fuel cell includes a metallic catalyst, and an electrocatalyst layer having a polymer A for hydrophobicizing the electrocatalyst layer and a uniform thickness of between 3 to 40 μm. The polymer A content is less than 10% by weight based on the metallic catalyst content. A method of producing a gas diffusion electrode for a PEM fuel cell and a method of hydrophobicizing a gas diffusion electrode include screen printing a paste onto a carrier and removing the screen-printing medium by heating. The paste includes at least one metallic catalyst with a content of polymer A up to at most 10% by weight, and a screen-printing medium. The electrocatalyst layer of the electrode has a significantly lower content of the catalyst inhibitor TEFLON® because it is not added only to the screen-printing paste but is subsequently applied, with the same surface-specific effect, by dipping the finished electrocatalyst layer in a solution containing TEFLON®.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/DE99/02622, filed Aug. 20, 1999, which designatedthe United States.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a gas diffusion electrode for use in PEMfuel cells and to a method for its production. The production method isintended, in particular, to make possible hydrophobicization of the gasdiffusion electrode.

[0004] The core of a PEM fuel cell is a membrane electrode unit that isbuilt up from a membrane with an electrode that is coated on both sidesand includes an electrocatalyst layer. The electrode normally has asolid, gas-permeable and electrically conductive carrier (e.g., carbonfabric or carbon paper), which is preferably hydrophobicized with apolymer suspension (in the following text, the polymer will be calledpolymer A, which here concerns polymers such as PTFE, i.e.,polytetrafluoroethylene, TEFLON®). Applied to the carrier is anelectrocatalyst layer that, in turn, is again hydrophobicized.Therefore, the polymer A can be contained both in the carrier and in theelectrocatalyst layer. In addition, the electrode can contain a furtherpolymer as binder, which, in the present connection, is designatedpolymer B.

[0005] Previously, the necessary content of polymer A forhydrophobicizing the electrocatalyst layer has generally been 20-60% byweight. A higher content of polymer A, such as TEFLON®, inhibits theactivity of the platinum catalyst, increases the content resistance, andreduces the porosity of the electrode. See Watanabe, J. Elektroanal.Chem. 195 (1985) 81-83. In other words, it has a detrimental effect onthe system. Therefore, the polymer A for hydrophobicizing theelectrocatalyst layer can also be referred to as a “catalyst inhibitor”.

[0006] In the prior art electrode, in addition to having a high contentof polymer A for hydrophobicizing the electrocatalyst layer (20-60% byweight and always based on the content of metallic catalyst), thehomogeneity of the thickness of the electrocatalyst layer also presentsa problem. There is a requirement to provide a suitable productionmethod that cost-effectively permits a uniform coating of the carrierwith dry catalyst powder in low layer thicknesses of 3-40 μm, andfurther permits the coating to be mass produced.

[0007] According to conventional methods (Watanabe, J. Electroanal.Chem. 195 (1985) 81-83; J. Elektroanal. Chem. 197 (1986) 195-208 M.Uchida, J. Elektrochem. Soc., 142 (1995) 463-468), a dry powderedmixture of catalyst powder previously hyrophobicized with PTFE ispressed onto the likewise hydrophobicized carrier. To produce themixture, the carbon powder is first mixed intensively with PTFEdispersion and then dried at a temperature above 280° C. In the process,the surface-active wetting agent (Triton X 100) contained in thedispersion is removed. The wetting agent is used to compensate for thepoor processing properties that arise from the high content of polymer Ain the screen-printing paste. The mixture is then pulverized. The methodis very complicated, and a uniform thickness of the electrocatalystlayer in low layer thicknesses may be produced technically only withgreat difficulty and in low numbers. In addition, disadvantages with themethod include:

[0008] a high content of polymer A for hydrophobicizing theelectrocatalyst layer is contained; and

[0009] for the purpose of processing, a wetting agent must be added,which has to be removed specifically and leaves behind interferingresidues.

[0010] The prior art discloses gas diffusion electrodes for use inelectrochemical cells. See, for example, U.S. Pat. No. 4,568,442 toGoldsmith, and U.S. Pat. No. 4,615,954 to Solomon et al. In such a case,the surface of a gas diffusion electrode is to be hydrophobic, with apolymer proportion of 30% being viewed as suitable. See, in particular,the example in U.S. Pat. No. 4,615,954 to Solomon et al.

[0011] According to U.S. Pat. No. 4,229,490 to Frank et al., andEuropean Patent Application 0 357 077 A1, the production of such gasdiffusion electrodes is carried out by a screen-printing technique.Screen printing is a conventional technique for producing a uniformlythin layer. The use of screen printing to build up an electrochemicalsystem is already known. According to U.S. Pat. No. 4,229,290 to Franket al., for such a purpose, the screen-printing paste, which containsTEFLON® dispersion, graphite, and platinum black, must, in turn, haveadded to it more than 50% by weight of the wetting or dispersing agent“Triton X 100” for the purpose of stabilization. The proportion ofTEFLON® used for hydrophobicization in the screen-printing paste, and,therefore, that which is present in the resulting electrocatalyst layeris about 25% by weight. The paste is printed onto a solid carrier, forexample, carbon paper, which again contains 60% by weight of TEFLON®.The result is a total content of TEFLON® of about 85%. The drawback withthe electrode produced by such a method, in addition to the extremelyhigh content of polymer A for hydrophobicizing the electrocatalyst layer(here: TEFLON®), is also the wetting agent added to more than 50% byweight (of the catalyst paste).

SUMMARY OF THE INVENTION

[0012] It is accordingly an object of the invention to provide a gasdiffusion electrode and a method for its production that overcomes thehereinafore-mentioned disadvantages of the heretofore-known devices andmethods of this general type and that is cost-effective and capable ofmass production, and that achieves the hydrophobicization of the gasdiffusion electrode with a low proportion of polymer.

[0013] With the foregoing and other objects in view, there is provided,in accordance with the invention, a gas diffusion electrode for a PEMfuel cell including a metallic catalyst, and an electrocatalyst layerhaving a polymer A for hydrophobicizing the electrocatalyst layer, acontent of the polymer A being less than 10% by weight based on acontent of the metallic catalyst, and a uniform thickness of between 3to 40 μm.

[0014] With the objects of the invention in view, there is also provideda method of producing a gas diffusion electrode for a PEM fuel cell,including the steps of screen printing a screen-printing paste onto acarrier, the screen-printing paste including at least one metalliccatalyst with a content of polymer A up to at most 10% by weight, and ascreen-printing medium, and removing the screen-printing medium byheating.

[0015] The subject of the invention is a gas diffusion electrode for aPEM fuel cell having an electrocatalyst layer having a content ofhydrophobicizing polymer A of less than 10% by weight and a uniformthickness of the electrocatalyst layer of less than or equal to 20 μm.Also, the invention relates to a gas diffusion electrode that isproduced by a screen-printing process with a screen-printing paste thatincludes a polymer A content for hydrophobicizing the electrocatalystlayer of at most 10% (based on the content of metallic catalyst), atleast one metallic catalyst, and a high-boiling solvent. The inventionfurther relates to a method for producing a gas diffusion electrode inwhich, in the screen-printing process, a catalyst paste that includes atleast one metallic catalyst and a screen-printing medium is printed ontoan electrode and/or a membrane, and the screen-printing medium isremoved by heating in a following, second operation. The inventionrelates to a method for hydrophobicizing a gas diffusion electrode inwhich a ready-coated electrode is dipped into a solution of the polymerA for hydrophobicization. The invention also relates to using a gasdiffusion electrode according to the invention in a fuel cell.

[0016] In accordance with another feature of the invention, theelectrocatalyst conveyor and/or the screen-printing paste (based ontheir content of metallic catalyst) contain only 0.01 to 1% by weight,preferably, 0.05 to 0.5% by weight, particularly, 0.075 to 0.2% byweight, and, in particular, 0.1% by weight of polymer A forhydrophobicizing the electrocatalyst layer.

[0017] In accordance with an added feature of the invention, the polymerA for hydrophobicizing the electrocatalyst layer is TEFLON®, inparticular, an amorphous modification of TEFLON® that can be broughtinto solution.

[0018] In accordance with an additional feature of the invention, themetallic catalyst used is platinum black or platinum on carbon.

[0019] In accordance with yet another feature of the invention, thehigh-boiling solvent used in the screen-printing and/or catalyst pasteis an ester and/or a ketone and/or an alcohol, in particular, glycolicacid butyl ester, cyclohexanone, and/or terpineol.

[0020] According to one refinement of the invention, the catalyst paste,apart from the metallic catalyst and the high-boiling solvent, also hasadded to it as binder a polymer B, preferably, a polymer that can bebaked out to 400° C.

[0021] In accordance with a further feature of the invention, thecontent of the polymer A in the electrocatalyst layer forhydrophobicizing the electrocatalyst layer approaches zero, with zerobeing ruled out.

[0022] In accordance with yet a further mode of the invention, forexample, the polymer A can be omitted completely from thescreen-printing paste, the hydrophobicization of the finishedelectrocatalyst layer is carried out after the screen-printing coatingby dipping the complete electrode into a solution of thehydrophobicizing polymer A. The solution contains the polymer Apreferably at 0.01 to 1% by weight, particularly preferably, 0.05 to0.5% by weight, and, quite particularly, preferably 0.075 to 0.2% byweight, in particular, 0.1% by weight. Preferably, the solvent is aperfluorinated solvent like a completely fluorinated organic compoundthat, for example, can be produced by the electrochemical fluorinationof alkanes.

[0023] In accordance with yet an added mode of the invention, it isadvantageous if, following the hydrophobicization, the electrode isdried in a further operation, preferably at temperatures between 20° C.and 120° C.

[0024] In accordance with yet an additional feature of the invention, inorder to fill up the large pores and, therefore, to reduce the quantityof catalyst needed for complete coating, a carbon paste includingelectrically conductive carbon black and screen-printing medium is firstprinted onto the carrier. The printing produces the very firstscreen-printed coating of the carrier with carbon. Only following thedrying of the first screen-printed coating is the screen printing withthe—considerably more expensive—catalyst paste carried out.

[0025] In accordance with again another mode of the invention, thecarrier is precoated with a carbon paste of electrically conductivecarbon black prior to the screen-printing step.

[0026] In accordance with again a further mode of the invention, inorder to achieve a different content of polymer A in the gas diffusionelectrode, both the carbon paste of the first screen-printing operationand the carrier, or both, can additionally contain polymer A.

[0027] The total content of polymer A in the gas diffusion electrode isconceptually separated from the critical content of “polymer A forhydrophobicizing the electrocatalyst layer” because the designationlisted is understood to mean only the quantity of polymer A that isapplied to the electrocatalyst layer by the dip bath and/or through thescreen-printing paste. The total content of polymer A in the gasdiffusion electrode (that is to say, the content of polymer A in thecarrier, in the first screen-printed layer, and in the electrocatalystlayer together) advantageously adds up to up to 20% by weight,preferably, to less than 15% by weight, particularly preferably, to lessthan 10% by weight, quite particularly preferably, to less than 5% byweight and, in particular, to less than 3.5% by weight.

[0028] The polymer A preferably used is TEFLON®, in particular, amodification that is present in amorphous and/or transparent form andmay be dissolved completely in fluorinated solvents. Alternatively,however, a different polymer, such as ethylene propylene copolymer or adifferent fluorine-containing polymer, e.g., polyvinylidene fluoride(PVDF) can also be used.

[0029] The electrocatalyst layer referred to here is the layer that ispreferably applied to a solid, gas-permeable and electrically conductivecarrier of the electrode, and on whose catalytic surface the anodicoxidation of the fuel to protons or the cathodic reduction of the oxygentakes place. The electrocatalyst layer includes at least the metalliccatalyst, which preferably contains platinum and can be used in pureform as platinum black or in diluted form as platinum on carbon in thecatalyst paste. The electrocatalyst layer preferably contains no furtherconstituents because, according to the preferred embodiment of theinvention, the screen-printing medium that is added to the catalystpaste for processing has been removed by drying and heating thefinished, that is to say, coated, electrode.

[0030] The “uniform electrocatalyst layer thickness” referred to here isa layer 3-40 μm thick, which has been applied by a conventionalscreen-printing process and whose thickness fluctuation is generallybelow that which can be achieved with a different coating technique forfuel-cell electrodes.

[0031] For processing, the screen-printing paste (also called carbon orcatalyst paste, depending on the operation) has added to it at least ahigh-boiling solvent as a screen-printing medium, such as an ester,ketone, and/or an alcohol, in particular, glycolic acid butyl ester,cyclohexanone, and/or terpineol.

[0032] It is advantageous if, as a screen-printing medium, it is notonly a high-boiling solvent that is added but also, as a binder, apolymer B, such as polyvinyl alcohol and/or polyethylene oxide.Preferably, the polymer B can be baked out, in particular, attemperatures up to 400° C., or leaves behind only residues that do notinterfere with the operation of the fuel cell.

[0033] The electrode is a gas-permeable, electrically conductive layeron the membrane, which preferably includes a carrier with anelectrocatalyst layer. The carrier or substrate used is, preferably, acarbon fabric or a carbon paper or another porous and electricallyconductive substrate.

[0034] Other features and modes that are considered as characteristicfor the invention are set forth in the appended claims.

[0035] Although the invention described herein as embodied in a gasdiffusion electrode and a method for its production, it is neverthelessnot intended to be limited to the details given because variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0036] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying examples.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] In the following text, the method according to the invention willbe explained in more detail using a preferred embodiment.

[0038] In order to produce the screen-printing pastes, the carbon orcatalyst powder is added to a screen-printing medium, made ofpolyethylene oxide dissolved in terpineol, for example, while stirring.The content of binder is 0 to 20% by weight, preferably, 5 to 15% byweight. The catalyst used is platinum black or platinum on carbon.Screen printing is carried out with a commercially availablescreen-printing machine. Stainless-steel screens with a size of up to760*700 mm² are used, with a mesh width of 100 to 300 meshes per inch(about 39 to 118 meshes per cm). Using the latter, wet layer thicknessesfrom 6 to 60 μm per printing operation can be achieved. Virtually anydesired areas are coated per printing operation, limited by the size ofthe printable area of the screen-printing machine. Following theprinting operation, the electrodes are dried at 120° C. and baked out at360° C. in order to remove the binder.

[0039] The platinum covering, determined by weighing is 2-3 mg/cm² ifpure platinum black is used as the catalyst and 0.15 to 0.4 mg/cm² ifplatinum on carbon is used as the catalyst, depending on the platinumcovering of the carbon.

[0040] For hydrophobicization, the ready-coated gas diffusion electrodeis dipped into a solution of a polymer A for hydrophobicizing theelectrocatalyst layer and is then dried. Any desired gas diffusionelectrode can be hydrophobicized retrospectively in this way.

[0041] Current/voltage curves of membrane/electrode units with gasdiffusion electrodes according to the invention were recorded, in whichan extremely low voltage drop at high current intensities could beobserved. The drop can be attributed, inter alia, to the low diffusioninhibition, caused by the low content of polymer A and damage to thehydrophobicization by residues of wetting agent within the porouselectrocatalyst layer.

[0042] The present screen printing method makes it possible to reducethe costs for electrode production considerably. Using thescreen-printing process, a uniform layer thickness is achieved over theentire electrode, even in the case of large electrodes (e.g., 36*36cm²), as well as good reproducibility during mass production. Becausethe hydrophobicization is carried out only at the conclusion of themethod, if at all, by dipping the complete electrode into a solution ofthe polymer A, the processing properties (and the bake-out behavior) ofthe screen-printing pastes are not impaired by the polymer suspensionand additional wetting and dispersion agents, which tend to coagulateand/or foam.

[0043] According to the invention, in order to hydrophobicize theelectrode, considerably lower quantities of polymer A are needed in theelectrocatalyst layer because the polymer A is deposited from thesolution only as a thin film on the surface of the electrode particles(carbon, platinum etc.). The electrocatalyst layer advantageouslycontains only 0.01 to 0.5% by weight, preferably, 0.05 to 0.3% byweight, particularly preferably, 0.075 to 0.2% by weight, and, inparticular, 0.1% by weight of polymer A for hydrophobicizing theelectrocatalyst layer, instead of 20 to 60% by weight as hitherto. As aresult, blockage of the gas pores by polymer A agglomerates in theelectrocatalyst layer and/or in the carrier are prevented to the maximumextent.

[0044] The invention replaces the previous hydrophobicization techniquein gas diffusion electrodes of fuel cells. Instead of the conventionalincorporation of the polymer A (which is a catalyst inhibitor) forhydrophobicizing the electrocatalyst layer in the electrocatalyst paste,the ready-coated electrode is dipped into a hydrophobicization bath. Theparticular advantage of such a gas diffusion electrode is, in additionto having a low polymer A content, also an improved homogeneity of thelayer thickness because the electrocatalyst paste can be processedbetter in the screen-printing process without the addition of polymer A.

We claim:
 1. A gas diffusion electrode for a PEM fuel cell, comprising:a metallic catalyst; and an electrocatalyst layer having: a polymer Afor hydrophobicizing said electrocatalyst layer, a content of saidpolymer A being less than 10% by weight based on a content of saidmetallic catalyst; and a uniform thickness of between 3 to 40 μm.
 2. Thegas diffusion electrode according to claim 1 , wherein saidelectrocatalyst layer has a uniform thickness of between 3 and 20 μm. 3.The gas diffusion electrode according to claim 1 , wherein said contentof said polymer A is between 0.01 and 1% by weight.
 4. The gas diffusionelectrode according to claim 1 , wherein said content of said polymer Aapproaches zero, said electrocatalyst has a surface, and said polymer Ais disposed on said surface of said electrocatalyst layer.
 5. The gasdiffusion electrode according to claim 1 , wherein said content of saidpolymer A approaches zero, and including a substrate, said polymer Abeing disposed on said substrate.
 6. The gas diffusion electrodeaccording to claim 1 , wherein said content of said polymer A approacheszero and said electrocatalyst has a surface, and including a substrate,said polymer A being disposed on at least one of said substrate and saidsurface of said electrocatalyst layer.
 7. The gas diffusion electrodeaccording to claim 1 , wherein said metallic catalyst includes one ofplatinum black and platinum on carbon, and a polymer B as binder.
 8. Amethod of producing a gas diffusion electrode for a PEM fuel cell, whichcomprises: screen printing a screen-printing paste onto a carrier, thescreen-printing paste including: at least one metallic catalyst with acontent of polymer A up to at most 10% by weight; and a screen-printingmedium; and removing the screen-printing medium by heating.
 9. Themethod according to claim 8 , wherein the screen printing step isperformed by screen printing the screen-printing paste having a uniformthickness of between 3 to 40 μm.
 10. The method according to claim 8 ,wherein the screen printing step is performed by screen printing thescreen-printing paste having a uniform thickness of between 3 and 20 μm.11. The method according to claim 8 , wherein the content of the polymerA is between 0.01 and 1% by weight.
 12. The method according to claim 8, wherein the content of the polymer A approaches zero and the polymer Ais disposed on at least one of the carrier and a surface of thescreen-printing paste.
 13. The method according to claim 8 , wherein thescreen-printing medium is a high-boiling solvent.
 14. The methodaccording to claim 8 , wherein the removing step is performed byremoving the screen-printing medium by heating to at most 400° C. 15.The method according to claim 8 , which further comprises precoating thecarrier with a carbon paste of electrically conductive carbon blackprior to the screen-printing step.
 16. The method according to claim 8 ,wherein the carrier is a substrate containing polymer A.
 17. The methodaccording to claim 8 , wherein the screen-printing paste has aproportion of polymer A approaching zero, and which further compriseshydrophobicizing an electrode by dipping the electrode ready-coated inthe screen-printing process into a polymer A solution.
 18. The methodaccording to claim 17 , which further comprises drying the electrodeafter hydrophobicization.
 19. In a fuel cell, a gas diffusion electrode,comprising: a metallic catalyst; and an electrocatalyst layer having: apolymer A for hydrophobicizing said electrocatalyst layer, a content ofsaid polymer A being less than 10% by weight based on a content of saidmetallic catalyst; and a uniform thickness of between 3 to 40 μm.
 20. Agas diffusion electrode for a PEM fuel cell produced in accordance withthe method of claim 8 , the gas diffusion electrode comprising: ametallic catalyst; and an electrocatalyst layer having: a polymer A forhydrophobicizing said electrocatalyst layer, a content of said polymer Abeing less than 10% by weight based on a content of said metalliccatalyst; and a uniform thickness of between 3 to 40 μm.
 21. The gasdiffusion electrode according to claim 1 , wherein said electrocatalystlayer has a uniform thickness of between 3 and 20 μm.
 22. The gasdiffusion electrode according to claim 1 , wherein said content of saidpolymer A is between 0.01 and 1% by weight.